EGR has proven to be an effective technique to reduce nitrous oxide (NOx) emissions by lowering combustion temperatures. The inert exhaust gas displaces oxygen in the combustion chamber and absorbs heat from combustion. The ability to lower combustion temperatures is increased when the recirculated exhaust gases are cooled before being introduced into the combustion chamber. In spark ignited, throttled engines EGR reduces pumping losses by allowing higher intake manifold pressures at lower engine speeds and loads. In external EGR systems, a portion of exhaust gases are recirculated to the intake manifold where they are reintroduced to combustion chambers along with intake air. In a high pressure EGR system the pressure of exhaust gases to be recirculated from the exhaust manifold must be higher than intake manifold pressure. This positive pressure difference between exhaust and intake manifold pressure is not always present throughout the load/speed range of an engine's operation. To enable EGR under these circumstances there are passive and active techniques to achieve the positive pressure difference.
Passive methods include employing passive restrictions that introduce a reduction in the cross-section of the flow geometry. For example, a restriction in the piping in the exhaust system can be employed to raise the back pressure in the exhaust manifold. A restriction can be formed in the exhaust manifold exit before exhaust gases enter a turbocharger, or the restriction can be the entry into the turbocharger itself. By restricting the flow area in such ways the pressure in the exhaust system is increased. Active methods include employing active restrictions, where there is a variable valve to change the cross-sectional flow geometry, and active pumping techniques. For example, an active pumping technique includes feeding exhaust gases upstream of a turbocharger compressor inlet such that they are compressed (pressurized) along with intake air and output to the intake manifold. A direct EGR pumping system employing, for example, a roots-type positive displacement pump is another active technique to increase exhaust gas pressure.
U.S. Pat. No. 4,179,892, issued Dec. 25, 1979 to Hans Heydrich, discloses an exhaust gas recirculation technique for a turbocharged internal combustion engine. The exhaust manifold of the engine is separated into two sections. A first section of the exhaust manifold is directed towards the large scroll of a twin-scroll turbocharger. A second section of the exhaust manifold is split and feeds both the small scroll inlet of the twin-scroll turbocharger and an EGR circuit. To create sufficient back pressure in the EGR circuit the small scroll is designed to provide a restriction of flow into the turbocharger. Since both the EGR circuit and the small scroll inlet of the turbocharger are fed from the same section of the exhaust manifold, the restriction provided by the small scroll inlet must be large enough to create the required back pressure, but an adverse result is that this reduces turbocharger and engine pumping efficiency.
U.S. Pat. No. 6,347,619, issued Feb. 19, 2002 to Whiting et al., discloses an exhaust gas recirculation system for a turbocharged engine. Each cylinder has a primary exhaust valve in fluid communication with an exhaust manifold, and a secondary exhaust valve in fluid communication with an EGR manifold. The EGR manifold is in fluid communication with the intake manifold through either a cold start EGR valve or an EGR cooler/valve combination. The timing of the opening of the secondary exhaust valves is such that the pressure in the EGR manifold is maintained higher than the pressure in the intake manifold. The exhaust manifold is routed to the inlet of a turbocharger. In some engine applications the exhaust valves from different cylinders can be open at the same time, for example overlapping at the end of the power stroke in one cylinder and the end of the exhaust stroke in another cylinder. Since exhaust gases from all the cylinders feed the turbocharger inlet, the exhaust gas flowing from cylinders that fire adjacently interfere with each other thereby reducing turbocharger efficiency.
The state of the art is lacking in techniques for high pressure exhaust gas recirculation systems. There is a need for a method and apparatus for improving high pressure exhaust gas recirculation in turbocharged internal combustion engines.